Why not?  But the former client might still owe a fee to the first preparer, who as with any contract can choose how to pursue collection. 

Good point.  But how would they be able to transmit the return with the former preparer PIN credentials and signature on it.  Wouldn't that be illegal?

They could not e-file the pdf. They could mail the pdf, IF the preparer name is printed in the preparer signature area. That printed signature is considered a signature. As Bob stated, there could be a contractual violation, as well as a moral one. 

You, as another preparer could use that pdf to reconstruct the return and do the e-file. You would also be responsible for the content of the return as created by you. 

Adds perspective - thanks! Are exempt organizations still allowed to mail returns? I thought that IRS was no longer accepting paper returns from exempt organizations.

Chrome AI says "In most cases, if you mail your Form 990 instead of e-filing, the IRS will reject it and consider the return not filed. The Taxpayer First Act of 2019 made electronic filing mandatory for most tax-exempt organizations for tax years beginning after July 1, 2019. "

This does not say that they can't mail it. 

I think they may accept a paper return and send a bill for a penalty for not efiling

Thanks